(a) Field of the Invention
The present invention relates to electrolyte compositions and lithium secondary batteries thereof, more particularly to electrolyte compositions containing monomers of conductive polymers which are capable of forming conductive polymer film on a surface of a positive electrode, and lithium secondary batteries thereof.
(b) Description of the Related Art
With the recent proliferation in the use of portable electronic devices, coupled with advancements made enabling increasingly smaller sizes and weights for these devices, research is being actively pursued to improve the energy density capabilities of lithium secondary batteries. These portable electronic machines have made it necessary to develop lithium secondary batteries with high energy density.
In the past, although lithium metal was used as the anode active material in lithium secondary batteries, a serious problem of dendritic formation on the surface of the lithium metal resulted during charging and discharging. This may cause short circuits, or more seriously, it could lead to the explosion of the battery. To prevent such problems, carbonaceous material is now widely used for the negative active material.
For cathode active materials in secondary batteries, metal chalcogenide compounds, enabling insertion and separation of lithium ions, are generally used, i.e. composite metal oxides such as LiMn2O4, LiMnO2, LiCoO2, LiNiO2, and LiNi1-xCoxO2 (0<x<1). The Mn-based active materials, LiMn2O4, and LiMnO2, can be easily synthesized, are less expensive than the other materials, and have minimal negative affects on the environment. However, the capacities of these materials are low. In particular, LiMn2O4 has a relatively low discharge capacity as compared to other lithiated transition metal oxides such as LiCoO2 and LiNiO2. Furthermore, when cycled at high rates of charge and discharge operations, the discharge capacity is excessively reduced. Additionally, when the charge and discharge operations are continuously performed at high temperatures, manganese ions readily elute into the electrolyte from the surface of LiMn2O4. These manganese ions seriously deteriorate the cycle life characteristics of the battery. LiCoO2 has been commercialized by Sony Energy Tec. as it exhibits an electrical conductivity at room temperature, provides a high level of battery voltage, and has exceptional electrode characteristics, even though it is unsafe when charging or discharging at high rates, and is more costly than the other materials. LiNiO2 has a high discharge and charge capacity and is the least expensive of the above active materials for positive electrodes, but is not easily synthesized.
In addition, the choice of suitable electrolytes is one of the factors for improving cell characteristics because reactions of electrodes and electrolyte have an effect on cell performance. The electrolyte systems have previously only played a role of transfer of lithium ions. These electrolytes can not prevent the large amount of current resulting from overcharge or feed-through which can cause heat explosions. Recently, use of additives capable of forming a solid electrolyte interface (SEI) on a surface of negative electrode has become known. However, electrolyte that prevents a large current by forming a conductive polymer film on a surface of a positive electrode has not been disclosed.